Generally, window fog detecting apparatuses for vehicles are mainly classified into a humidity detection type and an optical type. In the window fog detecting apparatus of the humidity detection type, for example, window fog is estimated by comparing a glass temperature with a dew-point temperature of ambient air thereof. The dew-point temperature is calculated based on outputs of a humidity sensor and an air temperature sensor, which are arranged in a passenger compartment of a vehicle.
The glass temperature is detected by various methods, such as a contact detection using a temperature sensor arranged on an inner surface of a glass, a non-contact detection using a infrared sensor, a detection based on a change of a resistance of a conductive thin film enclosed in a glass, and an estimation according to a calculation based on a vehicle outside temperature, a vehicle speed, a vehicle inside temperature and the like. The method of detection using the conductive thin film is described, for example, in Japanese Unexamined Patent Publication No. 2004-191249. The method of estimation according to the calculation is described, for example, in Japanese Patent No. 3309528.
Also, as another example of the window fog detecting apparatus of the humidity detection type, the window fog is determined by converting a relative humidity of air inside of a passenger compartment into a relative humidity on a glass surface (hereafter, a glass surface relative humidity). The fog detections described as above are performed to provide vehicles with the following effects, for example.
First, a dehumidifying operation of a refrigerant cycle of an air conditioner, that is, an operation of a compressor is performed in such a range that fog does not occur on the window glass. Therefore, the operating ratio of the dehumidifying operation is reduced, and power for driving the compressor is saved. This results in a reduction of the fuel consumption of a vehicle engine.
Second, a fog-restricting operation of the air conditioner is improved by performing the window fog determination. That is, the window fog is effectively reduced. Further, under a low temperature in winter or the like, a ratio of inside air sucked in the air conditioner is increased in such a range without causing the window fog. Thus, the ventilation heat loss is reduced and hence a heating performance of the air conditioner improves.
However, the above discussed glass temperature detections except for the contact detection using the temperature sensor are likely to increase costs. Also, detection accuracy is likely to vary due to estimation. Even in the contact detection in which the glass temperature is directly detected, a circuit board is likely to largely receive a stress depending on the mounting structure of the temperature sensor and the circuit board. Further, detection accuracy of the glass temperature is likely to be affected by contact structure between the glass surface and the temperature sensor.